A plug loosened during this operation, releasing a substantial quantity of crude oil into the shaft. That in turn led to the vaporisation of large volumes of hydrocarbon gas.
According to the investigation report from the Petroleum Safety Authority Norway (PSA), this incident could have developed into a major accident under slightly different circumstances. That would have involved extensive pollution and possible loss of life.
Pumping the oily water into the sea was done to protect safety on the platform, but it left a thin film of oil on the surrounding sea. Of the 217 people on board at the time, 156 were evacuated to the B and C installations, leaving only safety crew in place.
Crude oil flows with the aid of gravity down through pipes in the utility shaft from coolers on the cellar deck to the storage cells in the concrete gravity base structure (GBS).
These cells are partly filled with ballast water taken from the sea, and this is pumped out as crude flows in. This allows production to be continuously transferred to the storage cells, and oil to be periodically removed from there to shuttle tankers.
Crude is held on Statfjord A in 15 cells with a combined capacity of 1.2 million barrels. Each set of three is equipped with a common crude oil manifold for storage and transfer.
As oil is removed, the cells take in with ballast water to ensure that they are always filled with liquid – oil, seawater or both. That maintains platform stability and helps to withstand structural stresses on the GBS.
Crude and ballast water do not mix in the cells because the former is lighter and therefore floats on top. However, an unstable water/oil emulsion – or sludge – forms at the interface between the two fluids.
A dedicated system comprising pumps and piping was originally connected to the oil loading facilities leading in and out of the storage cells to transfer sludge between them.
This comprised three manifolds and valves to isolate it from the oil loading manifold. The valves suffered significant internal leaks, and the sludge system has never been used.
Sequence of events
Corrosion was found in the sludge system during 2004. To reduce the possibility of leaks, it was resolved to remove its manifold and as many connections to the oil loading system as possible.
The job of planning a method in detail was allocated to Aker Solutions, which in turn sub-contracted one of the assignments to IndustriKonsult. This work was to be done with the aid of hot tapping, a conventional technique for making connections to piping while it remains in operation.
A hole would be drilled in each pipe connection linking the sludge and loading systems, the flow of oil temporarily blocked by inserting a plug, and the piping cut off. The operation was completed by welding on a blind flange. But the hot taps were to done where the sludge piping was bent – a new job for everyone involved. When one of the operators sought to adjust a saw support – or set screw – on the hot tap tool, they screwed it too far out so that it fell off the thread and left an open hole.
Oil poured out directly from the storage cells into the shaft. To stop the leak, ballast water had to be pumped out of the storage cells while pumping water into the piping system. It took seven hours and 40 minutes before the flow had declined sufficiently for the saw support to be screwed back into position.
The oil flowed to the bottom of the utility shaft, where some oil/water sludge was pumped to the sea. Vaporisation from the oil led to the build-up of an explosive atmosphere in the shaft. About 150 tonnes of oil leaked out, and 50-70 tonnes of this was pumped to the sea. Standby ships equipped with the necessary equipment recovered about 20 tonnes.
The accident resulted in a four-day production shutdown on Statfjord A and to some extent on Snorre A. Vigdis had a one-day shutdown.
Actual and potential consequences
If the explosive blend of air and fumes which formed in the utility shaft had been ignited, it could have led in the worst case to a powerful blast with a major accident potential.
The fact that the incident did not have a more serious outcome was down to luck, and the PSA’s subsequent report reviewed both actual and potential consequences. What actually happened was that oil flowed into the utility shaft and a substantial volume of hydrocarbon vapour formed in the same place. Personnel were both splashed with oil and exposed to the fumes, but no evidence was found afterwards that individuals had suffered harm. The vapour also spread in lower concentrations to other parts of the platform. A substantial volume of oily water was discharged to the sea.
More serious were the potential consequences if circumstances had been marginally different, not least for the two operators directly involved in the work. They could have suffered major injury or died from breathing in the vapour, and could have been seriously hurt or killed if the fumes had ignited. In addition, other work was due to begin soon afterwards lower down the shaft and these operators would have been exposed in the same way.
Had the vapour ignited or built up, an explosion could have occurred with extensive damage to the platform and a number of people might have been killed or injured. The PSA report found the threat of a total utility shaft collapse to be relatively small. But the quarters module located above this shaft would have been directly exposed to an explosion. Crew on the platform were praised for devising an ad hoc solution to limiting the leak in the shaft by filling the pipe with water to displace the oil.
This method was not developed in advance and depended heavily on the system knowledge and understanding of individuals. Had they not been present, the spill could have been much larger.
The immediate cause of the accident was the absence of a stop arrangement or warning which prevented the saw support from being screwed too far out. Operators on the shift were not aware that a risk existed of screwing this device off, and no shut-off system or other measures to limit the size or duration of a leak had been established. Both the PSA investigation and StatoilHydro’s internal inquiry also addressed the underlying reasons for the accident.
The most important criticism in the PSA report was that the operator had not identified the major accident potential posed by the activity and made this clear to those doing the job. Nor had the company complied during execution of the project with the requirements of its own work processes, which describe such aspects as roles and responsibilities. This led in part to a failure to involve the right specialists and to identify deficiencies in design and methods. StatoilHydro had also done too little to fulfil its responsibility for ensuring the compliance of other players.
Aker Solutions, as the main contractor, also came in for its share of criticism. The PSA maintained that the company had not fulfilled its responsibility for managing the assignment. That included the choice and development of equipment and method, as well as securing sufficient knowledge of and compensating for the risk involved. IndustriKonsult, as the developer of the equipment used, had also failed to comply with its own or official requirements related to qualifying this hardware. Nor had it ensured adequate training for its own personnel.
A major oil spill to the sea had occurred on Statfjord A only six months earlier, in December 2007. On that occasion, the hose between the loading buoy and shuttle tanker Navion Britannia had broken. Some 27 500 barrels of oil had leaked to the sea.
Ranked as the second largest in Norwegian history, this spill had provoked massive criticism from environmental organisations and others – particularly in relation to a zero discharge guarantee StatoilHydro had given for operations in the far north.
The Bellona environmental association maintained that the oil leaks on Statfjord A should mean that the company was excluded from new licences in Norway’s 20th licensing round.
Union officials in StatoilHydro were also sceptical about safety after the incident, and argued that the discharge after the leak showed that the company had to spend more on maintenance. The unions used the incident to attack the new organisation due to be introduced by the company in 2009, which included building up maintenance teams on land. They claimed that having expertise ashore was not enough – it also had to be available offshore, particularly on such a complex structure as Statfjord A. It had been through so many changes that people who knew their way around were needed on board.
After the incident, StatoilHydro, Aker Solutions and IndustriKonsult were served with orders by the PSA to review their routines and identify measures which could yield improvements. On the basis of the PSA report, the police also investigated the accident on the basis that it could have developed into a major accident in marginally different circumstances.
Petroleum Safety Authority Norway. Investigation of incident. Hydrocarbon leak in utility shaft on Statfjord A 24 May 2008. Activity no 001037004.
StatoilHydro. Granskningsrapport. Intern ulykkegranskning. Oljelekkasje i utstyrsskaft på Statfjord A 24.5.2008. Report no A EPN L1 2008-4.
Petromagasinet, no 3, 2008. ”Statfjord kunne havnet på havets bunn”.